1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst.
2. Description of the Related Art
Recently, emission controls on automobiles and the like have been strengthened. Therefore, in order to address this issue, various exhaust gas-purifying catalysts for purifying hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOX) and the like in exhaust gas have been developed.
Many exhaust gas-purifying catalysts contain a precious metal as a catalytic metal. The precious metal plays a role in accelerating oxidation reactions of HC and CO and reduction reactions of NOX.
However, the precious metal has a problem that they are susceptible to poisoning by HC in the exhaust gas. When the precious metal is poisoned by HC, NOX reduction reaction on the precious metal becomes difficult to occur. Therefore, a catalyst comprising the precious metal sometimes shows an insufficient NOX purification efficiency specifically under a rich atmosphere in which an HC concentration in the exhaust gas is high.
Therefore, in order to suppress this, a technique comprising adding a salt of an alkaline-earth metal element to a catalyst comprising a precious metal has been suggested (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-207183). When such a composition is adopted, poisoning of the precious metal by HC is suppressed by the action of the alkaline-earth metal element, whereby exhaust gas purification performances such as an NOX purification efficiency can be improved to some extent.
However, there is room for further improvement of exhaust gas purification performances of exhaust gas-purifying catalysts.